Nancy Pelosi is beaming as she bustles into the House of Representatives dining room on Capitol Hill, which combines the busyness of a staff cafeteria with the grace of a late 19th-century reception room. A large painting of George Washington on the battlefield hangs on the wall, gazing across a full room of congressmen and women and their advisers and guests.

The smile might seem odd, as she has come straight from the House floor, where the Democrats have lost another vote. It is a predictable enough event in the Republican-dominated chamber but Pelosi marks down the vote on the country’s labour standards board as a victory by a different standard. “We didn’t lose a single Democrat. That was very big for us,” she says, taking pride that not one member of her party had voted with the Republicans.

In Congress, if a measure is certain to be defeated, leaders sometimes give members leeway to break the party line so that they can vote in a way that helps them in their districts. This is not a practice that sits well with Pelosi. “Desertion?” she says. “Would ‘desertion’ be the word?”

With Congress in session, often sitting from morning until late at night, she has little leeway in her choice of venue for lunch, a short walk down a flight of stairs from her office.

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Lunch with the FT

With her press secretary trailing behind, she cuts a slender, polished figure in a ruby-red trouser-suit as she leads the way to a vacant table in the noisy centre of the room. It appears that she hasn’t asked for, or been given, a special spot.

Pelosi, now 73, has long been a kind of liberal piñata for Republicans, a ready-made hate figure for conservatives seeking to conjure a leftwinger embodying the values they detest. When I mention her designation as a “liberal” soon after sitting down, she holds the palm of her hand to her chest, and says: “Proudly!”

She doesn’t shrink from the label but it vastly undersells her story. As speaker of the House for four years, from 2007 to 2011, she is not only the most senior elected woman politician in the US ever, she has also been a brutally effective party leader, marshalling consensus in a way few manage in Congress. Without her, President Barack Obama’s signature legislative achievements, such as healthcare reform, would have never become law.

Though firmly identified with the liberal views of San Francisco, just as formative an influence on Pelosi was her upbringing in Baltimore, the gritty eastern port city north of Washington where her father, Thomas D’Alesandro Jr, was a congressman and, later, mayor. Nancy, born in 1940 and the youngest of seven children (one died as an infant), grew up with five elder brothers in a household in which political campaigning was a part of life.

At home the D’Alesandros kept a “favour file”, a box of index cards recording the lists of people her father had helped out and who could repay the debt come polling day. The young Nancy learnt her lesson well. “Don’t think Nancy’s from San Francisco,” a longtime colleague, the late Pennsylvania congressman John Murtha, once said. “She’s from Baltimore.”

The connection with San Francisco came with her marriage in 1963 to Paul Pelosi, whom she met while both were at college. Once settled in the city, she had five children in six years. She gradually shifted from being a full-time housewife into political activism, hosting fundraisers at the couple’s home and winning spots on party committees, locally and then nationally. It wasn’t until 1987, at the age of 47 and when her youngest child was in high school, that she was elected to Congress.

Today, largely courtesy of her husband’s business success in real estate and finance, Pelosi is ranked the 10th wealthiest person in Congress with a fortune estimated at $94m, according to the Centre for Responsive Politics.

. . .

Proudly liberal and relentlessly partisan, Pelosi seems to have changed little since she arrived in the capital. But the make-up of her party has been transformed: the Democrats were once a bastion of white men, like much of the establishment in the rest of the country. Today, Pelosi presides over a caucus of 201 members of whom fewer than half are white men; the figure for the Republicans is 90 per cent.

The statistic reveals a lot about modern US politics. These days, white working-class men – the kind of people who put Pelosi’s father in office – mostly vote Republican. Obama won last year with support from women, minorities, young people and professionals. The president also prospered by backing gay marriage, something that would have been unthinkable only four years ago.

As we study the menu and waiters hover, I say to Pelosi that her comment about that morning’s vote reminds me that the US is the only country I can think of where “parliamentary” is used almost as a swearword. In parliaments, crossing the floor to vote against your party is relatively rare. In Congress, coalitions across the aisle were once common but are less so now, as partisanship hardens. The change has annoyed many coalition-building congressional traditionalists. For them, the word “parliamentary” conveys a sense of a party dictating to its members.

“I’m glad you brought that up because I do think about it quite a bit,” she says. “We’re not a parliamentary system. The party of the president – some people vote with him, some people don’t. That would be unthinkable in a parliamentary system.”

But how then to explain how Democrats, in her case, increasingly vote along party lines? “We don’t go in and say, ‘Here’s the thing.’ We develop it from the members up and we build our consensus so we all go down the path together.”

She says this gently, as though it is the most natural thing in the world for her members to follow her lead, and with her trademark eyes-wide-open smile. But she omits to mention the big sticks she carries that help her get her way: her phenomenal fundraising ability (she has raised $335m since 2002, putting her as an individual second only to presidential candidates in pulling in money) and her overview of the all-important positions on congressional committees, which all, one way or another, pass through her office. “Some come up through the seniority system but I’ve had to, shall we say, weigh in from time to time,” she says, smiling again.

It’s time to order. It is American Samoa week in the members’ dining room but that looks far too heavy on the cholesterol. Pelosi orders a Cobb salad and a cheese steak hoagie, or large filled roll. I plump for Maryland crab cakes, a reliable speciality of nearby Baltimore.

Pelosi is still holding forth on how the Democratic caucus has been transformed. “It doesn’t mean we’re better than white men. It just means we have more diversity in our thinking and that validation is really important,” she says. “Republicans are trying to figure out how to speak to women and minorities. Well, we have women and minorities as the majority of our caucus.’’

Indeed, when she arrived in Congress in the late 1980s, when HIV/Aids was at its most deadly in San Francisco, one of the first issues Pelosi took up was how to tackle the virus. Battling something that rightwingers called the “gay plague” wasn’t a popular position to hold at the time.

“The first thing on the TV shows that they would ask me [then] was: do you support gay marriage? I would say, ‘Yes.’ I was proud of it,” she says. “But back then it was like, ‘We’re going to label you right from the start so everyone knows how left and liberal you are.’ And now we have a president of the United States supporting it. It’s a beautiful thing.”

I ask her what she learnt from her father, even though, she admits, he never expected her to go into politics. Her elder brother Tommy was the anointed one in the family and later followed in his father’s footsteps to become Baltimore’s mayor. “But what I did learn from my father, as he planned elections and all the rest, is how to count votes.”

So does she have a formula to calibrate the truthfulness of members’ promises to vote in a certain way? “No. They would only not come through once,” she says, smiling brightly, again, and holding up a single index finger, seemingly to emphasise that people soon learn to level with her. “If they say, ‘I’d love to be with you,’ or, ‘This looks good to me’ – zero. The one thing I do conservatively is count. The members know not to say they’re going to be for something and then not do it.

“There’s a story [about counting] I like to tell of a little boy in school. The teacher says, ‘What is one and one?’ He says, ‘Two.’ ‘Two and two.’ ‘Four.’ She says, ‘Good.’ The boy says, ‘No, not good. Perfect.’ So 218 is our perfect number.” (In a chamber with 435 members, 218 is a majority.)

The butter-wouldn’t-melt-in-her-mouth tone glosses over the wrenching work involved in getting votes to pass Obama’s health bill in 2009. Pelosi even managed to pass an emissions trading bill, although it foundered in the Senate. I say she helped save Obama’s first-term agenda.

“Will you go tell him that?” she says with a twinkle, before course-correcting. “I’m just teasing. We have a wonderful rapport with the president. We’re so proud of him.” In mid-May, at a New York fundraiser, Obama singled her out as “the person you want alongside you” in a foxhole. “We love Nancy,” he said.

The hoagie, with fries, arrives first but she only picks at her food. One veteran congressional correspondent told me she had never seen Pelosi eat. She likes to binge on dark chocolate, and that may be where she gets her energy; it certainly doesn’t seem to be from ordinary food.

When I ask for the high point of her career, she nominates leading the Democrats to take back the house in 2006. The low point came just four years later, in 2010, when the Republicans regained control of the chamber with a smashing victory. The country might have caught up with her on social issues, such as gay marriage, but the election loss that year was a reminder that economic management is still the most important issue.

As part of celebrations to mark the Financial Times’ 125th anniversary this year, Penguin is publishing Lunch with the FT: 52 Classic Interviews, which includes encounters with world figures from Angela Merkel to Zaha Hadid. The book is available as a hardback and ebook. For more on the anniversary, visit www.ft.com/12

When I ask Pelosi if she took this defeat personally, she seems momentarily startled but is interrupted by a colleague, the Virginia congressman Jim Moran, strolling past after lunch. Spotting her salad, Moran says: “That’s the first healthy thing I’ve seen you eat in months,” before turning to me and adding: “She only eats bacon and doughnuts, while all of us are trying to figure out how to eat a healthy diet.”

He is quickly gone, and Pelosi launches into a lengthy explanation of what happened in 2010. Obamacare, the emissions trading bill, the “lies” spread by the Republicans, and then the failure of a bill to force business lobbies to disclose the source of foreign donations. “As soon as that failed, the money just poured in,” she says, into Republican coffers, although she doesn’t say where from.

In a parliamentary system, a leader such as Pelosi would have probably lost her job after such a big defeat. But no serious challenger emerged to her leadership position – a sign of her dominance of the Democratic caucus. Nor did one come forward after Republicans kept the House in 2012, with the help of electoral boundaries largely redrawn in their favour. She maintains that her party performed well.

“In the last election we ... what’s a euphemism for ‘took out’?” she turns to her press secretary.

“Won?” I interject.

“We won 16 tough Republican races. These were prize jewels of the Republican leadership. So we did really very well,” she says, even though most commentators had expected the Democrats to win some seats back.

Time is running short so I ask about China. Pelosi vehemently criticised both George Bush Sr and Bill Clinton for not being tougher on the country’s human rights record. The Chinese, in return, once described her as, “the most hated person in China”. She laughs at the memory. “Wow. I’m really getting there,” she remembers thinking, throwing back her arms in mock triumph. Relations between Pelosi and Beijing have mellowed and she and Chinese officials have a regular dialogue on issues including climate change, long a pet topic of hers. “We’ve been fighting so long, we’re pals,” she jokes.

I ask if, after 25 years in Washington, she has any plans to retire but she fends the question off. “I came here to be here about 10 years,” she says, before talking about everything that still needs to be done, notably childcare for working women and tackling income inequality.

The bill comes and, in a break with FT tradition, we split it. Since this paper’s parent company, Pearson, employs lobbyists in Washington, I cannot pay for her under congressional ethics rules. “I’m going to be a big tipper,” she says, adding a generous top-up, and we are soon on our way. Walking to the stairs, with a slightly stiff gait, is the only moment she seems anything like her 73 years.

I ask her what Congressman Moran meant with his quip about her eating habits. “He gets annoyed when I have bacon on top of a chocolate doughnut in the morning,” she replies.